Field of the Invention
The present invention relates to a slip control device which is provided in an electric automobile such as a vehicle running by drive of only a motor or a vehicle including both a motor and an internal combustion engine, and which performs torque limitation when a slippage occurs during running with the motor, to eliminate the slippage.
Description of Related Art
In order to prevent acceleration from being stopped due to a slippage of a drive wheel caused by an excessive driving torque during acceleration of a vehicle, a traction control device for the vehicle is generally known to detect the slip amount of the drive wheel and limits engine output or a wheel braking force such that the slip amount of the drive wheel reaches a target slip amount for the frictional coefficient of a road surface (e.g., Patent Document 1). In particular, the traction control device controls the slip amount by decreasing the engine output or increasing the braking force.
The following technique has been proposed for an electric automobile which performs slip control (Patent Document 2). In this proposal, a slip ratio λ is obtained on the basis of rotation frequencies N1 and N2 of respective driven and drive wheels. Then, a generated torque estimation value Te due to an external force applied to the vehicle, such as the force exerted by the vehicle body weight, is obtained by a disturbance observer. An entire action torque T acting on the drive wheel is obtained on the basis of the generated torque estimation value Te and a motor torque Tm, and a coefficient of friction μ between a road surface and a tire is estimated on the basis of the torque and the slip ratio λ. A permissible maximum torque Tmax is obtained on the basis of the coefficient of friction μ and a vertical load FZ, and torque limitation is performed such that the torque does not exceed the permissible maximum torque Tmax.